1. Field of the Invention
The present invention relates to a post-polymerization process for modifying the molecular weight distribution of polymers having an initial monomodal molecular weight distribution to produce polymers having a bimodal molecular weight distribution.
2. Description of Related Art
Some polymers such as polyethylenes or copolymers of ethylene with up to about 50 mole % of propylene or butene-1 are known to undergo a molecular weight increase when subjected to shear mixing in the presence of a free radical initiator such as an organic peroxide. This molecular weight increase may be intensified by the additional inclusion of polyfunctional (polyunsaturated) grafting agents. The use of such polyfunctional agents is recommended by manufacturers for the peroxide curing or crosslinking of copolymers of ethylene and propylene having a high ethylene content, e.g., at least about 50 mole % of ethylene.
Other polymers such as crystalline polypropylene, copolymers of propylene with up to 10 mole % ethylene, polyisobutylene, copolymers of a C.sub.4 to C.sub.7 isomonolefin with up to 10 wt. % of isoprene or up to 20 wt. % para-alkylstyrene undergo a molecular weight decrease when subjected to shear mixing, especially in the presence of a free radical initiator, as disclosed for example in U.S. Pat. Nos. 3,862,265 and 4,749,505.
It is believed that the distinction between these mechanisms is that an extensive polymer chain network is developed in the high ethylene content polymers when treated as described above, whereas chain scission without any extensive polymer chain network development takes place when the other polymers are sheared, alone or in the presence of a free radical initiator.
Polymers having a bimodal molecular weight distribution have useful properties. The higher molecular weight species within the bimodal distribution imparts higher melt strength properties to the polymer while the lower molecular weight species within the bimodal distribution imparts better processing characteristics to the polymer.
Routes to such polymers typically involve incorporation of a small amount of a crosslinking agent into the polymerization process. In the manufacture of polybutadiene, divinylbenzene is used for this purpose (W. Hofman, "Rubber Technology Handbook", Hanser Publishers, New York, 1989, p. 55). Divinylbenzene is also used to produce "precrosslinked" nitrile rubber, which can be blended with normal nitrile rubber to create a polymer with a bimodal molecular weight distribution (W. Hofman, "Rubber Technology Handbook", Hanser Publishers, New York 1989, p. 69). Another polymer may be used as the crosslinking agent as in the production of star branched butyl polymers, as disclosed in U.S. Pat. No. 5,071,913.
Heretofore, no technique has been disclosed for a post-polymerization process for both reducing the extent of molecular weight decrease of polymers which normally undergo molecular weight decrease when subjected to shear mixing and providing a polymer having a bimodal molecular weight distribution.